The present invention relates to valve metals. More particularly, the present invention relates to methods of nitriding valve metal having an oxide layer, and to the nitrided valve metal material so produced.
Capacitors in general, and tantalum capacitors in particular, have been a major contributor to the miniaturization of electronic circuitry. Tantalum capacitors also operate over a wide temperature range and have good shelf life, long service, and are useful in extreme environments. Tantalum capacitors are typically manufactured by compressing tantalum powder into a pellet and then sintering the pellet to form a porous body. The porous body is then anodized in a suitable electrolyte to form a continuous dielectric oxide film on the sintered body. The pores are filled with an electrolyte or counter electrode and a lead wire is attached to form the capacitor.
The performance characteristics of capacitors or electrodes formed from capacitor grade powders are expressed in terms of specific charge and electrical current leakage. The specific charge is a measure of electrical charge capacity of the capacitor and is usually proportional to the surface area of the powder as a sintered and anodized pellet. The electrical leakage is an indication of how well the capacitor holds the specific charge. Capacitors with improved electrical leakage characteristics are recognized as having higher reliability.
It is well known that the performance characteristics of finished capacitors are affected by the chemical and physical characteristics of the base material used to make the capacitors. Base powders can develop undesirable crystalline-like flaws in the dielectric film during sintering and anodic oxidation. The flaws allow current to pass through the dielectric, producing parts that exhibit excessive electrical leakage and premature failure. The likelihood of flaw formation increases for high voltage capacitors where anodization voltages of 100 V or higher are used.
Various attempts have been made to improve the performance characteristics of finished capacitors by reacting small quantities of modifying agents to the base material. A range of additives or “dopants” have been used including nitrogen, silicon, phosphorous, boron, carbon, and sulfur. Attempts also specifically include forming a uniform tantalum nitride film on powder by subjecting the base powder material to ammonia gas and mixtures of nitrogen or ammonia with an inert gas such as argon. Alternatively, the base powder material may be exposed to a salt bath consisting of NaCN, NaCO3, BaCl2, and KCl. The doped base powder material then can be pressed into anodes and sintered. The problem with this process of doping the powders is that a minimum effect is achieved, and when the powders are pressed or otherwise formed into an anode and used as a capacitor, the dielectric layer still diffuses into the metal anode causing destabilization of the anode.
Accordingly, a need exists for a method of improving chemical and physical characteristics of base material used to make the capacitors.